1. Field of the Invention
The present disclosure relates generally to a downhole nuclear magnetic resonance (NMR) apparatus, data processing, and interpretation methods for evaluating a characteristic of a region, and particularly for detecting and quantifying a light hydrocarbon-bearing earth formation in a subterranean region.
2. Description of the Related Art
Exploration and production of hydrocarbons generally requires that a borehole be drilled into an earth formation. The borehole provides access to the earth formation for performing measurements related to a property of the formation. Many wireline and logging-while-drilling (LWD) tools probe the formation in the shallow region of radial depths surrounding the borehole. The information gathered by these tools in the shallow radial depths of investigation often are contaminated by invasion of drilling fluid, also known as mud filtrate invasion. In order to accurately quantify hydrocarbon saturation and predict multiphase flow in the earth formation, petrophysicists are interested in determining the hydrocarbon and invading filtrate fluid volumes and saturations as a function of radial depth. The variation of mud filtrate saturation is often more pronounced in gas reservoirs due to the high mobility of gas. It is suspected that mud filtrate invasion may vary within a few inches in the flushed zone.
In the prior art, three approaches have generally been used to detect invasion variations and generate an invasion profile. The invasion profile is a correlation of depth to an amount of invasion. The first approach compares density porosity with NMR apparent porosity corresponding to depth of invasion (and, thus frequency). The first approach requires knowledge of the matrix density and, therefore, is subject to the accuracy of the technique for determining the matrix density. Furthermore, since density porosity does not have a well defined depth of invasion, the variation of NMR apparent porosity only reflects the relative variation of flushed zone gas saturation, not the absolute gas zone saturation. The second approach applies a linear constraint to simultaneous inversion of all frequency data obtained from NMR measurements. The second approach works only if the invasion variation is consistent to the constraint but will be less effective for the case where gas replenishes the flushed zone long after the mud cake builds up. An independently measured gas saturation profile is desired. The third approach acquires NMR T2 distribution logs corresponding to a plurality of depths of investigation and observes the differences between the logs.
Three approaches using NMR are also generally used for the detection of gas. The first approach is based on T1 contrast between a slowly relaxing gas and a fast relaxing liquid. The first approach is less effective for gas zones having water in very large pores and/or containing light oil or oil-based-mud-filtrate (OBMF). The second approach is based on T1/T2app contrast where apparent T2 relaxation time, T2app, is reduced from intrinsic T2 relaxation time, T2intr, due to diffusion effect, which causes additional diffusion-induced decay in a magnetic gradient environment. Since the diffusivity of gas is much higher than that of liquid phase hydrocarbon and water, the ratio for gas is much larger than that for the liquids. The third approach is based on a hydrogen index effect.
The gas detection methods based on T1 or T1/T2app usually require a long data acquisition time for measuring NMR signal build-up at multiple stages of polarization. Compared to CPMG T2 measurement, T1 is an inefficient method in terms of the amount of data in a unit of time. Thus, usually the logging speed has to be reduced for the T1 log. As a result, T1 logging is more useful in LWD than in wireline logging. In order to increase the logging speed, a multi-frequency NMR tool is used such that within the time waiting for the NMR tool to polarize protons in the zone of investigation back to the full-polarization state, the NMR tool can acquire data at different frequencies, corresponding to different depths of investigation. By distributing different wait times among these frequencies, one can successfully acquire multiple stages of polarization among the different frequencies and then process the data together as if the data were obtained from the same polarization build-up. This last process excludes significant variations in formation properties or in an invasion zone. For a relatively narrow range of depths of invasion and for low-mobility fluids, the variation of invasion may be insignificant. Occasionally, the high mobility gas may cause slight variation of invasion within a few inches.
Unlike T2 measurements, T1 logging data provides the intrinsic relaxation time because diffusion induced decay,
                                          1                          T                              2                ⁢                diff                                              =                                                                      (                                      γ                    ·                    G                    ·                    TE                                    )                                2                            ·              D                        12                          ,                            (        1        )            affects only T1 measurements. To reduce the diffusion effect on T2 measurements, one must reduce G·TE, which is subjected to limitations of the NMR tool.
Therefore, what are needed are techniques for acquiring NMR data efficiently and yet providing a relaxation time substantially close to the intrinsic relaxation time.